Issue 57

E. Sgambitterra et alii, Frattura ed Integrità Strutturale, 57 (2021) 300-320; DOI: 10.3221/IGF-ESIS.57.22

An electro-mechanical testing machine (MTS Criterion s42, USA) equipped with a 5 kN load cell has been used. All the experiments were carried out in displacement control mode (0.05 mm/min) and curved loading platens with 42.5 mm of radius, according to the ISRM [57], were used (Fig. 10). Six polymeric samples were investigated for statistical aim and all of them were airbrushed with white and black paint in order to get a proper pattern with a random grey scale distribution. Fig. 10 reports an image of the speckle pattern of the sample together with the intensity histogram. Samples were made with a diameter of 55 mm ad a thickness of 8 mm in order to get plane stress conditions, according to the Timoshenko theory [56]. Correlation analyses were performed by using a subset size of 61 pixels and a subset distance of 5 pixels. In addition, tensile tests were performed on the same material in order to calculate the effective elastic properties; three dog-bone shaped samples with 8 mm of thickness and 19 mm of width, according to ASTM D638, were used to perform the tests. HBM K-XY3 strain gauges were glued at the center of the samples and the HBM QuantumX MX1615B Strain Gauge Bridge Amplifier together with the Catman software was employed to record longitudinal and transverse strains. The crosshead speed was set to 1 mm/min and the Poisson ratio, ν , was calculated taking into account the transverse sensitivity of the strain gauges and the Poisson ratio of the material adopted for their calibration.

Figure 10: Schematic depiction of the Brazilian disk together with the load equipment suggested by the ISRM standard. Case study 3 The last case study consists in the measurement of the coupling pressure generated by a SMA ring when it is thermally activated on axisymmetric components. SMA rings exploit temperature-induced phase transformation mechanisms to offer new ways to join and seal cylindrical components as demonstrated by some recent research and/or technological studies as well as by a significant number of patents [58-62]. A 38NiCrMo3 steel ring, simulating the pipe system, was clamped with an external commercial Ni46.5Ti45.0Nb8.5 SMA ring. These metal rings are supplied in the expanded condition. Once shrunk, they apply a uniform gripping pressure that is onto the substrate. Thanks to the thermal hysteresis of the material, the radial force is kept during the subsequent cooling of the assembly to room temperature, which represent the operating condition of the coupler. Fig. 11 illustrates the geometry and dimension of the NiTiNb SMA ring used in this investigation, together with a schematic depiction of the SMA-steel ring assembly analyzed for contact pressure measurement. Figure also reports an image of the speckle pattern of the steel ring together with the intensity histogram. An initial SMA ring/steel rings diametric clearance of about 0.15 mm has been chosen. In order to prove the accuracy of the least square method in calculating the radial force generated by the SMA ring on the steel one, strain gauge (SG) measurements were also performed. In particular, x-y electrical strain gauges (XC11, HBM), having a wide operative temperature range (-200 °C/+250 °C), were mounted at the internal diameter of the steel ring (see Fig. 11) by using a two component epoxy resin (EP310S, HBM). Two strain gauges were glued at a relative angle of 120 °C (see Fig. 11) and quarter bridge configurations were adopted, which gives four different signals from the two x-y SGs. In order to delete the apparent

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